Thanks to a promising new way of measuring DNA’s age, researchers from University of California Los Angeles have developed a biological clock that may, for the first time, estimate the chronological age of cells with relative accuracy.

Scientists have used body substances like saliva and studied chromosome changes as markers for aging, but the UCLA team found that methylation, a process that dividing cells use to make subtle changes in the way DNA is expressed, can more accurately determine a cell’s — and therefore a person’s — age, without the help of a birth certificate.

During methylation, cells pull in methyl groups to sit on top of DNA, which can impair or enhance the activity of that section of DNA. So Steve Horvath, a professor of human genetics at the David Geffen School of Medicine at UCLA and his team figured that methylation would be a good stand-in for how old cells were. They compared methylation in healthy and cancerous human tissue in 8,000 samples of 51 types of tissues and cells, including those in the heart, lungs, brain, liver, and kidney. In their research, published in the journal Genome Biology, they highlighted 353 methylation markers that changed with age.

Using that data to build a statistical model of a biological clock that correlated methylation with chronological age, the scientists found that breast tissue ages faster than the rest of a woman’s body. Healthy breast tissue is typically about two years older than other tissues in the body, but in those with breast cancer, even healthy breast tissue that surrounds a tumor can be up to 12 years older than expected. Such accelerated aging may in part explain why breast cancer is so common among women, since cancer is a disease of aging cells. Heart tissue, on the other hand, as hard as it works to pump blood day in and day out throughout our lives, is about nine years younger than the researchers would have predicted based on their model.

“It’s surprising that one could develop a clock that reliably keeps time across the human anatomy,” Horvath said in a statement. He stresses, however, that for now, the connection between methylation and age is only that — an association, and not a causal link. “My approach really compared apples and oranges, or in this case, very different parts of the body: the brain, heart, lungs, liver, kidney and cartilage,” he says. The clock also did not work for all tissues, which suggests that more complex processes may be responsible for determining a particular cell’s chronological age.

The research does, however, hint that methylation may be an important part of aging, and teasing apart how cells regulate that process could help to explain aging, as well as suggest ways to intervene with anti-aging strategies.